Department of Agriculture, Bangkok, Thailand 10900
SummaryGlobal invasive plant may invade in some countries, but it is not invasive plant in its native country or in range of distribution. Those plants are utilized in some means, such as vegetable, fruit, ornamental plant or medicinal plant in it native country or even it was invasive plant but people learned how to manage with that plant, they can utilize it. On the contrary way, an invasive plant of any country may not be invasive in any country else. Most of the invasive plants are intentionally introduced to a country for some purpose especially for ornamental plant. Hydrocotyle umbrellata L. was introduced to Thailand as ornamental plant, the plant can grow well and with some habit of invasive plant of this plant, nowadays the plant was found in many natural areas as weed.

1. Introduction

Most of invasive plant in an area is alien plant which was introduced from different habitat within a country or other country across geographic barrier. Well development of transportation stimulates traveling and international trading which cause of increasing introduction of alien species. Some of introduced alien plants can establish and spread out if it can adapt itself to survive in the new habitat well. If the environment of new habitat is suitable for it growth, the plant may express it invasiveness and compete for space and nutrient with native plant which it may complete over native plants due to lack of it natural enemy in new habitat. If the invasive plant invades to new area widely it may become noxious weed in that area. In the past there were many invasive plants become noxious weeds out side it native range such as Water hyacinth, Giant sensitive plant, Pennisetum grasses. They were introduced to many countries intentionally for some purposes such as ornamental plant, erosion control, soil improvement, forage fodder.

2. Global invasive plants in Thailand and its status

The Global Invasive Species Database which developed by Invasive Species Specialist Group (ISSG) of the World Conservation Union / Species Survival Committee (IUCN/SSC) under the Global Invasive Species Programme (GISP) had listed 388 species as invasive species. Among these, there are 205 plant species which are 8 algae/seaweed, 33aquatic plants, 1 fern, 26 grass, 35 herb, 2 palm, 1 sedge, 50 shrub, 27 tree, and 22 vine/climber.

Among those 205 of global invasive plants 71 are exist in Thailand. They are 15 aquatic plants, 10 grasses, 6 herbs, 2 palms, 23 shrubs/trees, 13 vine/climbers and 2 shrubs/climbers. About 29 species are known as introduced plants, other are uncertain or no information about introduction. Most of the plants are utilized by some mean, ornamental plant, medicinal plant, etc. Only 10 species are weed without any utilization such as Arundo donax, Lygodium microphyllum, Neyraudia reynaudiana, Polygonum perfoliatum. Pennisetum polystachyon, P. clandestinum etc. (Table 1)

Since agriculture is one of the main task of Thai population. The plants invaded in agricultural area and becomes troublesome or weed for farmer are shown in Table 2

3. Case study of Hydrocotyle umbellata L.H. umbellata, common name as dollar weed, a member of Apiaceae, was introduced to Thailand as ornamental plant or aquarium plant more than 10 year ago. The plant is attractive for it shiny green round leaf (Fig. 1). Since the plant can propagate easily with stem. New shoot can grow from maxillary bud and seem to be no natural enemy in Thailand. It might be weed in near future. Experiments to evaluate weed potential on this plant were conduced in 2002. They are growth of Dollar weed and competition of Dollar weed and a native plant, Centella asiaitca.

Fig.1 Hydrocotyle umbrellata L.

The result shown that Dollar weed can grow well not only in aquatic condition but also in dry condition (Fig.2). The plant can produce new shoot from stem with young leaf if the stem was cut out from the mother plant by the cut part produce root and then can survive in the water and new branch germinate from the bud between stem and petiole (Fig 3). From 1 leaf at beginning of the experiment, the plant can grow up to 174 leaves within 141 day period. (Fig.4). The plant can compete over the native plant, C. asiatica, a member of same family (Fig.5 and Fig.6). These mean the plant has potential of weed.